Magnetic tape

ABSTRACT

A magnetic tape is disclosed, comprising a support having thereon a magnetic layer comprising ferromagnetic particles and a binder as the main components, (i) the magnetic tape being for use in a magnetic recording/reproducing system in which signals recorded are reproduced with a magneto resistive head, (ii) the ferromagnetic particles contained in the magnetic layer being hexagonal-ferrite magnetic particles having an average tabular diameter of from 10 to 40 nm, and the magnetic layer having a machine direction coercive force H c  of from 158 to 350 kA/m, a squareness ratio SQ⊥ as measured in the direction perpendicular to the magnetic plane of 0.5 or lower, and a thickness of from 30 to 300 nm.

FIELD OF THE INVENTION

[0001] The present invention relates to a magnetic tape of the coatedtype (i.e., a magnetic recording particulate tape) for high-densityrecording. More particularly, the present invention relates to amagnetic tape for use in a system in which signals are bidirectionallyrecorded/reproduced along the tape machine direction with a magnetoresistive head (MR head).

BACKGROUND OF THE INVENTIN

[0002] In the field of magnetic tapes, investigations are recently beingmade enthusiastically on magnetic tapes as external storages forrecording computer data (the so-called backup tapes), with the spread ofminicomputers, personal computers, and office computers such as workstations. Especially in magnetic tapes for serves where large quantitiesof data are dealt with, the so-called linear serpentine mode has beenput to practical use, in which signals are bidirectionallyrecorded/reproduced along the tape machine direction so as to heightenthe speed of data processing. For practically using magnetic tapes insuch applications, there is an especially strong desire for animprovement in recording capacity for attaining an increased recordingcapacity and a reduced recording-medium size, besides the desire for asize reduction and an increase in information-processing ability incomputers.

[0003] Magnetic recording media heretofore in wide use comprise anonmagnetic support and, formed thereover by coating, a magnetic layercomprising a binder and dispersed therein iron oxide, cobalt-modifiediron oxide, CrO₂, ferromagnetic metal particles, or hexagonal-ferriteparticles. Of these magnetic materials, fine particles of hexagonalferrites are known to have excellent high-density recordingcharacteristics. However, in recording/reproducing with an inductivehead which has hitherto been mainly used in the systems employingflexible media, the fine particles of a hexagonal ferrite have a lowsaturation magnetization and a sufficient output has not been obtainedtherewith.

[0004] In the removable recording employing such flexible media,however, the magneto resistive heads (MR heads) in use with hard diskshave come to be used.

[0005] Since MR heads have high sensitivity, a sufficient reproductionoutput is obtained even with use of fine particles of a hexagonalferrite. It is known that due to the noise reduction characteristic ofhexagonal ferrites, a high C/N ratio is obtained. For example, JapanesePatent Application (Laid-Open) Nos. 342515/1994 and 302243/1998 disclosea technique which employs fine particles of barium ferrite (BaFe) anduses an MR head for reproduction.

[0006] However, since the well-known hexagonal-ferrite fine particlessuch as those shown above not only have a tabular shape but have an axisof easy magnetization perpendicular to the plane, the tabular particlesare oriented, by calendering during the production of coated-typemagnetic tapes, so that the tabular planes become parallel to the tapemachine direction. As a result, the amount of magnetization componentsperpendicular to the magnetic plane is increased. The isolated pulsewaveform becomes asymmetrical due to the perpendicular magnetizationcomponents and the reproduction output hence changes with the directionof reproduction. Namely, the related-art magnetic recording mediumemploying fine particles of a hexagonal ferrite is unsuitable for use inrecording/reproducing in the linear serpentine mode.

[0007] Furthermore, even when that magnetic recording medium of therelated art is used in a mode other than the linear serpentine mode,such as, e.g., helical scan mode, the results are an increased peakshift in high-density recording due to waveform interference and thenecessity of waveform equalization.

SUMMARY OF THE INVENTION

[0008] An object of the present invention is to provide a magnetic tapewhich has satisfactory electromagnetic characteristics, is significantlyreduced in error rate especially in a high-density recording region, andcan be produced at low cost with excellent productivity.

[0009] Another object of the present invention is to provide a magnetictape of the coated type (i.e., a magnetic (recording) particulate tape)which, when used in a recording/reproducing system employing an MR head,shows excellent suitability for high-density recording with reducednoise.

[0010] The present invention provides a magnetic tape having thefollowing constitutions, whereby those objects of thepresent inventionare accomplished.

[0011] (1) A magnetic tape comprising a support having there on amagnetic layer comprising ferromagnetic particles and a binder as themain components,

[0012] (i) the magnetic tape being for use in a magneticrecording/reproducing system in which signals recorded are reproducedwith a magneto resistive head,

[0013] (ii) the ferromagnetic particles contained in the magnetic layerbeing hexagonal-ferrite magnetic particles having an average tabulardiameter of from 10 to 40 nm, and the magnetic layer having a machinedirection coercive force H_(c) of from 158 to 350 kA/m, a squarenessratio SQ⊥ as measured in the direction perpendicular to the magneticplane of 0.5 or lower, and a thickness of from 30 to 300 nm.

[0014] (2) The magnetic tape as described in (1) above which is for usein recording/reproducing in the linear serpentine mode.

[0015] In the magnetic tape of the present invention, hexagonal-ferritemagnetic particles which are fine particles and have a high Hc are usedto reduce the amount of magnetization components perpendicular to themagnetic plane (regulate the SQ⊥ to 0.5 or lower) and the thickness ofthe magnetic layer is regulated to 30 to 300 nm. As a result,reproduction can be conducted with diminished waveform distortion andreduced noise. This effect is markedly produced when the magnetic tapeis used in the linear serpentine mode in which signals arebidirectionally recorded/reproduced along the machine direction.

DETAILED DESCRIPTION OF THE INVENTION

[0016] [Magnetic Layer]

[0017] In the magnetic tape of the present invention, the magnetic layermay be formed on the support either directly or through a nonmagneticlower layer. Since the magnetic layer, which is for use with an MR head,has a reduced thickness, it is preferred to employ a multilayerconstitution including a nonmagnetic lower layer. The machine directioncoercive force H_(c) of the magnetic layer is from 158 to 350 kA/m(2,000 to 4,430 Oe), preferably from 170 to 280 kA/m. In case where themachine direction coercive force H_(c) of the magnetic layer is lowerthan 158 kA/m, the reproduction output is insufficient. In case wherethe machine direction coercive force thereof exceeds 350 kA/m, magnetichead saturation occurs, resulting in insufficient recording.

[0018] The magnetic layer desirably has such a magnetizationdistribution that the amount of components which undergo a (magnetic)flux revolution upon application of a magnetic field of 80 kA/m (1,000Oe) or lower is preferably smaller than 1%, more preferably 0.7% orsmaller, most preferably 0.5% or smaller.

[0019] The thickness of the magnetic layer is from 30 to 300 nm,preferably from 50 to 250 nm, most preferably from 50 to 200 nm.Thicknesses thereof smaller than 30 nm result in an insufficientreproduction output. In case where the thickness thereof is larger than300 nm, a phase difference arises between the magnetization componentpresent in inner parts of the layer and the magnetization componentpresent in surface parts of the layer, resulting in enhanced waveformasymmetry.

[0020] The squareness ratio SQ of the magnetic layer as measured in anin-plane direction is preferably from 0.6 to 0.95, more preferably from0.65 to 0.85.

[0021] The squareness ratio SQ⊥ as measured in the directionperpendicular to the magnetic plane is 0.5 or lower, preferably 0.4 orlower, more preferably 0.35 or lower. Although the lower limit of SQ⊥ istheoretically 0, it is vertically 0.1 or higher.

[0022] [Ferromagnetic Particles]

[0023] The ferromagnetic particles used in the magnetic layer in thepresent invention are particles of a hexagonal ferrite. Examples thereofinclude substitution products of barium ferrite, strontium ferrite, leadferrite, or calcium ferrite and cobalt substitution products. Specificexamples thereof include barium ferrite and strontium ferrite each ofthe magnetoplumbite type, magnetoplumbite-type ferrites whose surfacehas been coated with spinel, and magnetoplumbite-type barium ferrite andstrontium ferrite each partly containing a spinel phase. Such ferritesmay contain atoms of elements other than the given elements. Examples ofsuch optional elements include Al, Si, S, Sc, Ti, V, Cr, Cu, Y, Mo, Rh,Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au, Hg, Pb, Bi, La, Ce, Pr, Nd, P,Co, Mn, Zn, Ni, Sr, B, Ge, and Nb. In general, ferrites to whichelements such as Co—Zn, Co—Ti, Co—Ti—Zr, Co—Ti—Zn, Ni—Ti—Zn, Nb—Zn—Co,Sb—Zn—Co, or Nb—Zn have been added can be used. Some ferrites containpeculiar impurities depending on raw materials or production processes.

[0024] The term “average tabular diameter” used for thehexagonal-ferrite magnetic particles means the average of the tabulardiameters of the hexagonal plates. The average tabular diameter thereofis from 10 to 40 nm, preferably from 10 to 35 nm, more preferably from15 to 35 nm. Especially in the case where an MR head is used forreproduction in order to attain an increased track density, the tabulardiameter is preferably 35 nm or smaller because of the necessity ofnoise reduction. However, in case where the average particle diameterthereof is smaller than 10 nm, thermal fluctuations occur and stablemagnetization cannot hence be expected. On the other hand, averagetabular diameters thereof exceeding 40 nm result in increased noises andare hence unsuitable for high-density magnetic recording according tothe present invention. The average tabular ratio (arithmetic mean oftabular diameter/tabular thickness) of the magnetic particles isdesirably from 1 to 15, preferably from 1 to 7. Although small values ofthe average tabular ratio are preferred from the standpoint of attaininga higher packing property in the magnetic layer, sufficient orientationcannot be obtained therewith. On the other hand, values of the averagetabular ratio larger than 15 result in increased noises due to particlestacking.

[0025] When the magnetic particles have a size within that range, theparticles have a specific surface area as measured by the BET method(SBET) of from 10 to 100 m²/g. The specific surface area roughly agreeswith the arithmetic mean calculated from the particle tabular diameterand tabular thickness. The narrower distribution of tabulardiameter/tabular thickness of the particle, the more the magneticparticles are usually preferred. A numerical comparison can be made bymeasuring arbitrarily selected five hundred particles on a TEMphotograph of each particulate material. Although many particulatemagnetic materials do not have a normal distribution, the magneticparticles to be used in the present invention are ones whose coefficientof variance represented by standard deviation based on the calculatedaverage size (σ/average size) is generally from 0.1 to 2.0, preferablyfrom 0.1 to 1.0, more preferably from 0.1 to 0.5.

[0026] For obtaining magnetic particles having a narrow particle sizedistribution, a technique is being used in which a reaction system forparticle formation is kept as even as possible and the particles yieldedare subjected to a treatment for distribution improvement. Examples ofthis treatment include a method in which the particles are treated withan acid solution to selectively dissolve ultrafine particles therein.

[0027] The hexagonal-ferrite fine particles have an average particlevolume of generally from 1,000 to 10,000 nm³, preferably from 1,500 to8,000 nm³, more preferably from 2,000 to 8,000 nm³.

[0028] Magnetic materials having a coercive force H_(c) of about from 40to 400 kA/m can usually be produced. Although higher values of H_(c) areadvantageous for high-density recording, H_(c) is limited by thecapacity of the recording head to be used. In the present invention, theHc of the magnetic material is about from 120 to 360 kA/m, preferablyfrom 158 to 350 kA/m. When a head having a saturation magnetizationexceeding 1.4 T is to be used, it is preferred to employ a magneticmaterial having an H_(c) of 175 kA/m or higher. H_(c) can be regulatedby regulating particle size (tabular diameter/tabular thickness), thekind and amount of an element incorporated, the substitution site forthe element, reaction conditions for particle formation, etc. Thesaturation magnetization σ_(s) of the magnetic material is generallyfrom 40 to 80 A·m²/kg. The finer the particles, the more the σ_(s) tendsto become low. Well known techniques for improving σ_(s) include tocombine a magnetoplumbite ferrite with a spinel ferrite and to selectthe kind and amount of an element to be incorporated. It is alsopossible to use a W-type hexagonal ferrite. Furthermore, a technique isbeing used in which before a magnetic material is dispersed, the surfaceof the magnetic particles is treated with a substance suitable for thedispersion medium and polymer to be used.

[0029] As the surface-treating agent, is used an inorganic compound oran organic compound. Representative examples of such compounds includethe oxides or hydroxides of silicon, aluminum, and phosphorus, varioussilane coupling agents, and various titanium coupling agents. The amountof the surface-treating agent to be used may be from 0.1 to 10% based onthe magnetic material.

[0030] The pH of the magnetic material also is important for dispersion.The optimal value of pH is generally in the range of about from 4 to 12depending on the dispersion medium and polymer to be used. However, a pHof about from 6 to 11 is selected from the standpoints of the chemicalstability of the medium and storage stability. The water content of themagnetic material also influences dispersibility. A water content offrom 0.01 to 2.0% is generally selected although the optimal valuethereof depends on the dispersion medium and polymer to be used.Examples of processes for producing a hexagonal ferrite include: a glasscrystallization method which comprises mixing starting materials such asbarium oxide, iron oxide, an oxide of a metal with which iron is to besubstituted, and boron oxide as a glass-forming substance so as toresult in a desired ferrite composition, melting the mixture, rapidlycooling the melt to form an amorphous material, and subsequentlysubjecting the amorphous material to a heat treatment and then towashing and pulverizing to thereby obtain a crystalline powder of bariumferrite; a hydrothermal reaction method which comprises neutralizing asolution of metal salts having a barium ferrite composition with analkali, removing the by-product, subsequently heating the mixture in aliquid phase at 100° C. or higher, and then subjecting the mixture towashing, drying, and pulverization to obtain a crystalline powder ofbarium ferrite; and a coprecipitation method which comprisesneutralizing a solution of metal salts having a barium ferritecomposition with an alkali, removing the by-product, subsequently dryingthe mixture, treating the reaction product at 1,100° C. or lower, andthen pulverizing it to obtain a crystalline powder of barium ferrite.The hexagonal ferrite to be used in the present invention may be oneproduced by any process.

[0031] [Specific Technique for Attaining SQ⊥≦0.5]

[0032] For enabling the magnetic layer in the present invention to havea perpendicular-direction squareness ratio SQ⊥ within the range shownabove, the following technique can, for example, be used. A magneticlayer is formed on a nonmagnetic lower layer. This multilayer structureis subjected to magnetic orientation in the machine direction and thento calendering. In the calendering, the nonmagnetic lower layerfunctions as a cushioning layer to contract upon pressing by thecalendering rolls and thereby enable the magnetic material in the upperlayer to be less brought down. Thus, the perpendicular-directionsquareness ratio SQ⊥ of the magnetic layer can be regulated so as to bewithin the range shown above.

[0033] The lower layer serving such a function can be formed, forexample, by a method in which a binder having a glass transition pointlower than the calendering temperature is incorporated and/or a methodin which nonmagnetic particles having a high aspect ratio are used forforming voids prior to calendering. Either of these two methods or acombination of both may be used. Methods other than these may, ofcourse, be used.

[0034] In the case where a binder having a glass transition point lowerthan the calendering temperature is used in the lower layer, the glasstransition point of the binder is lower preferably by at least 10° C.,more preferably by at least 20° C., than the calendering temperature.

[0035] In the case where nonmagnetic particles having a high aspectratio are used in the lower layer, the aspect ratio of the nonmagneticparticles is preferably 3 or higher, more preferably 5 or higher, mostpreferably 7 or higher. The amount of the binder to be used for formingthe nonmagnetic layer is preferably from 5 to 50% by weight, morepreferably from 5 to 35% by weight, most preferably from 10 to 30% byweight, based on the nonmagnetic particles.

[0036] [Nonmagnetic Layer]

[0037] The nonmagnetic layer which may be formed as a lower layerbetween the support and the magnetic layer will be explained below indetail.

[0038] The lower layer is not particularly limited in constitution aslong as it is substantially nonmagnetic. However, it usually comprisesat least a resin. Preferred examples thereof include a layer comprisinga resin and particles, e.g., inorganic particles or organic particles,dispersed in the resin. Although the inorganic particles are usuallypreferably nonmagnetic particles, magnetic particles may be used as longas the lower layer is substantially nonmagnetic.

[0039] The nonmagnetic particles can be selected, for example, frominorganic compounds such as metal oxides, metal carbonates, metalsulfates, metal nitrides, metal carbides, and metal sulfides. Examplesof such inorganic compounds include α-alumina having an α-alumina havingan a-conversion rate of 90% or higher, β-alumina, γ-alumina, θ-alumina,silicon carbide, chromium oxide, cerium oxide, α-ironoxide, hematite,goethite, corundum, silicon nitride, titanium carbide, titanium oxide,silicon dioxide, tin oxide, magnesium oxide, tungsten oxide, zirconiumoxide, boron nitride, zinc oxide, calcium carbonate, calcium sulfate,barium sulfate, and molybdenum disulfide. These inorganic compounds maybe used alone or in combination of two or more thereof. Especiallypreferred are titanium dioxide, zinc oxide, the iron oxides, and bariumsulfate from the standpoints of the narrowness of particle sizedistribution and the availability of many techniques for functionimpartation. More preferred of these are titanium dioxide and a-ironoxide.

[0040] In the case of using nonmagnetic particles having a high aspectratio in the lower layer in order to regulate theperpendicular-direction squareness ratio SQ⊥ of the magnetic layer so asto be within the range shown above, the aspect ratio of the nonmagneticparticles is as shown above. Preferred examples of the nonmagneticparticles having a high aspect ratio include α-iron oxide, titaniumoxide, and alumina.

[0041] The particle size of the nonmagnetic particles is preferably from0.005to 2 μm. However, particulate nonmagnetic materials differing inparticle size may be used in combination according to need.Alternatively, a single particulate nonmagnetic material having awidened particle diameter distribution may be used so as to produce thesame effect. Especially preferred nonmagnetic particles have a particlesize of from 0.01 to 0.2 μm. Especially when the nonmagnetic particlesare a granular metal oxide, the average particle diameter thereof ispreferably 0.08 μm or smaller. When the nonmagnetic particles are anacicular metal oxide, the long-axis length thereof is preferably 0.3 μmor shorter, more preferably 0.2 μm or shorter. The tap density thereofis generally from 0.05 to 2 g/mL, preferably from 0.2 to 1.5 g/mL. Thewater content of the nonmagnetic particles is generally from 0.1 to 5%by weight, preferably from 0.2 to 3% by weight, more preferably from 0.3to 1.5% by weight. The pH of the nonmagnetic particles is generally from2 to 11, and is preferably in the range of from 5.5 to 10.

[0042] The specific surface area of the nonmagnetic particles isgenerally from 1 to 100 m²/g, preferably from 5 to 80 m²/g, morepreferably from 10 to 70 m²/g. The crystallite size of the nonmagneticparticles is preferably from 0.004 to 1 μm more preferably from 0.04 to0.1 μm. The oil absorption amount thereof as measured with DBP (dibutylphthalate) is generally from 5 to 100 mL/100g, preferably from 10 to 80mL/100g, more preferably from 20 to 60 mL/100g. The specific gravitythereof is generally from 1 to 12, preferably from 3 to 6. The particleshape thereof may be any of acicular, spherical, polyhedral, and tabularshapes. The Mohs' hardness thereof is preferably from 4 to 10. Thenonmagnetic particles have an SA (stearic acid) adsorption amount ofgenerally from 1 to 20 μmol/m², preferably from 2 to 15 μmol/m², morepreferably from 3 to 8 μmol/m². The pH thereof is preferably in therange of from 3 to 6. The nonmagnetic particles are preferably subjectedto a surface treatment to thereby cause Al₂O₃, SiO₂, TiO₂, ZrO₂, SnO₂,Sb₂O₃, ZnO, or Y₂O₃ to be present on the surface of the particles.Especially preferred are A120₃, SiO₂, TiO₂, and ZrO₂ from the standpointof enhancing dispersibility. More preferred of these are Al₂O₃, SiO₂,and ZrO₂. These may be used in combination of two or more thereof or maybe used alone.

[0043] The surface treatment may be conducted by suitable methodsaccording to purposes. For example, a surface treatment layer formed bycoprecipitation may be used. Alternatively, use may be made of a methodin which alumina is deposited first and this surface layer is thentreated with silica or a method in which the alumina treatment andsilica treatment are conducted in the reversed order. Although a poroussurface treatment layer may be formed according to purposes, it isgenerally preferred to form a surface treatment layer which ishomogeneous and dense.

[0044] Specific examples of the nonmagnetic particles for use in thelower layer include Nanotite manufactured by Showa Denko K.K.; HIT-100and ZA-G1 manufactured by Sumitomo Chemical Co., Ltd.; α-hematiteDPN-250, DPN-250BX, DPN-245, DPN-270BX, DPN-500BX, DBN-SA1, and DBN-SA3manufactured by Toda Kogyo Corp.; titanium oxide TTO-51B, TTO-55A,TTO-55B, TTO-55C, TTO-55S, TTO-55D, and SN-100 and a-hematite E270,.E271, E300, and E303 manufactured by Ishihara Sangyo Kaisha, Ltd.;titanium oxide STT-4D, STT-30D, STT-30, and STT-65C and α-hematite α-40manufactured by Titan Kogyo K.K.; MT-100S, MT-100T, MT-150W, MT-500B,MT-600B, MT-100F, and MT-500HD manufactured by Tayca Co., Ltd.;FINEX-25, BF-1, BF-10, BF-20, and ST-M manufactured by Sakai ChemicalIndustry Co., Ltd.; DEFIC-Y and DEFIC-R manufactured by Dowa Mining Co.,Ltd.; AS2BM and TiO₂ P25 manufactured by Nippon Aerosil Co., Ltd.; 100Aand 500A manufactured by Ube Industries, Ltd.; and nonmagnetic materialsobtained by burning these.

[0045] A carbon black may be incorporated into the lower layer, wherebythe well-known effects of lowering the surface electrical resistanceR_(s) and reducing the light transmittance can be produced and,simultaneously therewith, a desired value of micro-Vickers hardness canbe obtained. It is also possible to incorporate a carbon black into thelower layer to thereby enable the layer to have the effect of storing alubricant therein. Examples of the kinds of carbon blacks usable in thepresent invention include furnace black for rubbers, thermal black forrubbers, coloring black, and acetylene black. The carbon black to beincorporated into the lower layer should be one which has been optimizedwith respect to the following properties according to the desiredeffect. Use of a combination of two or more carbon blacks may produce anenhanced effect.

[0046] The carbon black to be incorporated into the lower layer has aspecific surface area of generally from 100 to 500 m²/g, preferably from150 to 400 m²/g, and a DBP absorption amount of generally from 20 to 400mL/100g, preferably from 30 to 400 mL/100g. The particle diameter of thecarbon black is generally from 5 to 80 nm, preferably from 10 to 50 nm,more preferably from 10 to 40 nm. The carbon black preferably has a pHof from 2 to 10, a water content of from 0.1 to 10%, and a tap densityof from 0.1 to 1 g/mL. Specific examples of carbon blacks usable in thepresent invention include BLACKPEARLS 2000, 1300, 1000, 900, 800, 880,700, and VULCAN XC-72 manufactured by Cabot Corp.; #3050B, #3150B,#3250B, #3750B, #3950B, #950, #650B, #970B, #8SOB, MA-600, MA-230,#4000, and #4010 manufactured by Mitsubishi Chemical Industries Ltd.;CONDUCTEX SC, RAVEN 8800, 8000, 7000, 5750, 5250, 3500, 2100, 2000,1800, 1500, 1255, and 1250 manufactured by Columbian Carbon Co.; andKetjen Black EC manufactured by Akzo N.V. Such carbon blacks may besurface-treated with a dispersant or the like or grafted with a resinbefore use. A carbon black whose surface has been partly graphitized mayalso be used.

[0047] Before being added to a coating solution, the carbon black may bedispersed in a binder. Those carbon blacks can be used in an amount of50% by weight or less based on the inorganic particles and in an amountof 40% by weight or less based on the total amount of the nonmagneticlayer.

[0048] Those carbon blacks can be used alone or in combination of two ormore thereof. With respect to carbon blacks usable in the presentinvention, reference may be made to, for example, Kâbon Burakku Binran(edited by Carbon Black Association, Japan).

[0049] Organic particles may be added to the lower layer according topurposes. Examples thereof include acrylic/styrene resin particles,benzoguanamine resin particles, melamine resin particles, andphthalocyanine pigments. Also usable are polyolefin resin particles,polyester resin particles, polyamide resin particles, polyimide resinparticles, and poly(fluoroethylene) resins. For producing such organicparticles, processes such as those described in Japanese PatentApplication (Laid-Open) Nos. 18564/1987 and 255827/1985 may be used.

[0050] Binder resins, lubricants, dispersants, additives, solvents,methods for dispersion, and others usable for forming the lower layermay be the same as those for the magnetic layer which will be describedlater. In particular, with respect to the amounts and kinds of binderresins and the amounts and kinds of additives and dispersants,well-known techniques for forming magnetic layers are applicable.

[0051] [Binder]

[0052] As the binder for the magnetic layer or lower layer in thepresent invention is used any of well-known thermoplastic resins,thermosetting resins, and reactive resins and mixtures of these. Thethermoplastic resins are ones having a glass transition temperature offrom −100 to 150° C., a number-average molecular weight of from 1,000 to200,000, preferably from 10,000 to 100,000, and a degree ofpolymerization of about from 50 to 1,000.

[0053] Examples of such thermoplastic resins include polymers orcopolymers comprising units of one or more of vinyl chloride, vinylacetate, vinyl alcohol, maleic acid, acrylic acid, acrylic acid esters,vinylidene chloride, acrylonitrile, methacrylic acid, methacrylic acidesters, styrene, butadiene, ethylene, vinyl butyral, vinyl acetal, andvinyl ethers as constituent units, and further include polyurethaneresins and various rubbery resins. Examples of the thermosetting resinsor reactive resins include phenolic resins, epoxy resins, curablepolyurethane resins, urea resins, melamine resins, alkyd resins,reactive acrylic resins, formaldehyde resins, silicone resins,epoxy-polyamide resins, mixtures of a polyester resin and an isocyanateprepolymer, mixtures of a polyester polyol and a polyisocyanate, andmixtures of a polyurethane and a polyisocyanate. These resins aredescribed in detail in Purasuchikku Handobukku, published by AsakuraShoten. Well-known electron beam-curable resins may be used for formingeachlayer. Examplesoftheseresinsandprocessesforproducing these aredescribed in detail in Japanese Patent Application (Laid-Open) No.256219/1987. The resins enumerated above may be used alone or incombination of two or more thereof. However, preferred examples thereofinclude combinations of at least one member selected from vinyl chlorideresins, vinyl chloride/vinyl acetate copolymers, vinyl chloride/vinylacetate/vinyl alcohol copolymers, and vinyl chloride/vinylacetate/maleic anhydride copolymers with apolyurethane resin, andfurther include combinations of the at least one member, a polyurethaneresin, and a polyisocyanate.

[0054] The polyurethane resin may have a well-known structure such as apolyester polyurethane, polyether polyurethane, polyether polyesterpolyurethane, polycarbonate polyurethane, polyester polycarbonatepolyurethane, or polycaprolactone polyurethane. For obtaining furtherimproved dispersibility and durability, it is preferred to use,according to need, one or more of the above-enumerated binders whichhave, incorporated therein through copolymerization or additionreaction, polar groups of at least one kind selected from —COOM, —SO₃M,—OSO₃M, —P═O(OM)₂, —O—P═O(OM)₂ (wherein M is a hydrogen atom or analkali metal), OH, NR₂, N⁺R₃ (R is a hydrocarbon group), an epoxy group,SH, CN, and the like. The amount of such polar groups is from 10⁻¹ to10⁻⁸ mol/g, preferably from 10⁻² to 10⁻⁶ mol/g.

[0055] Specific examples of those binders that can be used in thepresent invention include VAGH, VYHH, VMCH, VAGF, VAGD, VROH, VYES,VYNC, VMCC, XYHL, XYSG, PKHH, PKHJ, PKHC, and PKFE manufactured by UnionCarbide Corp.; MPR-TA, MPR-TA5, MPR-TAL, MPR-TSN, MPR-TMF, MPR-TS,MPR-TM, and MPR-TAO manufactured by Nisshin Chemical Co., Ltd.; 1000W,DX80, DX81, DX82, DX83, and 100FD manufactured by Denki Kagaku KogyoK.K.; MR-104, MR-105, MR110, MR100, MR555, and 400X-110A manufactured byNippon Zeon Co., Ltd.; Nippollan N2301, N2302, and N2304 manufactured byNippon Polyurethane Industry Co. Ltd.; Pandex T-5105, T-R3080, T-5201,BurnockD-400, D-210-80, Crisvon 6109, and 7209 manufactured by DainipponInk & Chemicals, Inc.; Vylon UR8200, UR8300, UR8700, RV530, and RV280manufactured by Toyobo Co., Ltd.; Daiferamine 4020, 5020, 5100, 5300,9020, 9022, and 7020 manufactured by Dainichiseika Kogyo K.K.; MX5004manufactured by Mitsubishi Chemical Industries Ltd.; Sunprene SP-150manufactured by Sanyo Chemical Industries, Ltd.; and Salan F310 and F210manufactured by Asahi Chemical Industry Co., Ltd.

[0056] In the case where a binder having a glass transition point lowerthan the calendering temperature is used in the lower layer in order toregulate the perpendicular-direction squareness ratio SQ⊥ of themagnetic layer so as to be within the range shown above, preferredexamples of the binder include polyurethane resins in view of the factthat calendering is usually conducted at temperatures in the range offrom 70 to 120° C.

[0057] The amount of the binder to be used for forming the nonmagneticlayer or magnetic layer is in the range of generally from 5 to 50% byweight (i.e. by mass), preferably from 10 to 30% by weight, based on thenonmagnetic particles or the ferromagnetic particles. In the case ofemploying a vinyl chloride resin, it is preferred to use the resin in anamount of from 5 to 30% by weight in combination with from 2 to 20% byweight polyurethane resin and from 2 to 20% by weight polyisocyanate.However, a polyurethane alone or a combination of a polyurethane and anisocyanate alone may be used, for example, when there is a possibilitythat a slight amount of chlorine might be released to cause headcorrosion. In the case of using a polyurethane in the present invention,this resin is desirably one having a glass transition temperature offrom −50 to 150° C., preferably from 0 to 100° C., more preferably from30 to 90° C., an elongation at break of from 100 to 2,000%, a stress atbreak of from 0.05 to 10 kg/mm² (0.49 to 98 MPa), and a yield point offrom 0.05 to 10 kg/mm² (0.49 to 98 MPa).

[0058] The magnetic tape of the present invention can have two or morecoating layers. Consequently, it is, of course, possible to form theindividual layers so that these layers differ from each other in binderamount, the proportion of a vinyl chloride resin, polyurethane resin,polyisocyanate, or another resin in the binder, the molecular weight andpolar group content of each resin contained in the magnetic layer, theaforementioned physical properties of a resin, etc., according to need.Rather than being thus regulated, these factors should be optimized foreach layer. For attaining this, well-known techniques concerningmultilayered magnetic layers are applicable. For example, in the case offorming layers having different binder amounts, an increase in binderamount in the magnetic layer is effective in diminishing the scratchesof the magnetic layer surface, while an increase in binder amount in thenonmagnetic layer is effective in imparting flexibility thereto andthereby improving head touching.

[0059] Examples of the polyisocyanate for use in the present inventioninclude isocyanates such as otolylene diisocyanate, 4,4′-diphenylmethanediisocyanate, hexamethylene diisocyanate, xylylene diisocyanate,naphthylene-1,5-diisocyanate, o-toluidine diisocyanate, isophoronediisocyanate, and triphenylmethane triisocyanate, products of thereactions of these isocyanates withpolyhydric alcohols,andpolyisocyanates formed through condensation of isocyanates. Theseisocyanates are commercially available under the trade names of:Coronate L, Coronate HL, Coronate 2030, Coronate 2031, Millionate MR,and Millionate MTL manufactured by Nippon Polyurethane Industry Co.,Ltd.; Takenate D-102, Takenate D-110N, Takenate D-200, and TakenateD-202 manufactured by Takeda Chemical Industries, Ltd.; and Desmodur L,Desmodur IL, Desmodur N, and Desmodur HL manufactured by Sumitomo BayerCo., Ltd. For each layer, these polyisocyanates may be used alone, orused in combination of two or more thereof, taking advantage of adifference in curing reactivity.

[0060] [Carbon Black, Abrasive Material]

[0061] The carbon black for use in the magnetic layer in the presentinvention may be furnace black for rubbers, thermal black for rubbers,coloring black, acetylene black, or the like. The carbon blackpreferably has a specific surface area of from 5 to 500 m²/g, a DBPabsorption amount of from 10 to 400 mL/100g, an average particlediameter of from 5 to 300 nm, preferably from 10 to 250 nm, morepreferably from 20 to 200 nm, a pH of from 2 to 10, a water content offrom 0.1 to 10%, and a tap density of from 0.1 to 1 g/ml. Specificexamples of carbon blacks usable in the present invention includeBLACKPEARLS 2000, 1300, 1000, 900, 905, 800, 700, and VULCAN XC-72manufactured by Cabot Corp.; #80, #60, #55, #50, and #35 manufactured byAsahi Carbon Co., Ltd.; #2400B, #2300, #900, #1000, #30, #40, and #10Bmanufactured by Mitsubishi Chemical Industries Ltd.; CONDUCTEX SC, RAVEN150, 50, 40, and 15, and RAVEN-MT-P manufactured by Columbian CarbonCo.; and Ketjen Black EC manufactured by Japan EC Co. These carbonblacks may be surface-treated with a dispersant or another agent orgrafted with a resin before use. A carbon black whose surface has beenpartly graphitized may also be used.

[0062] Before being added to a magnetic coating solution, the carbonblack may be dispersed in a binder. Those carbon blacks can be usedalone or in combination. In the case of using a carbon black, its amountis preferably from 0.1 to 30% based on the magnetic material. The carbonblack incorporated in the magnetic layer functions to prevent staticchange in the layer, to reduce the coefficient of friction of the layer,to impart a light-shielding property for the layer, and to improve thefilm strength. Such effects are produced to different degrees dependingon the kind of carbon black used. Consequently, it is, of course,possible in the present invention to properly use carbon blacksaccording to purposes so as to give an upper magnetic layer and a lowernonmagnetic layer which differ in the kind, amount, and combination ofcarbon blacks, on the basis of the aforementioned properties includingparticle size, oil absorption amount, electrical conductivity, and pH.Rather than being thus regulated, these factors should be optimized foreach layer. With respect to carbon blacks usable in the magnetic layerin the present invention, reference may be made to, for example, KâbonBurakku Binran (edited by Carbon Black Association).

[0063] Abrasive materials usable in the present invention are knownabrasive materials mostly having a Mohs' hardness of 6 or higher.Examples thereof include α-alumina having an α-alumina having anα-conversion rate of 90% or higher, β-alumina, silicon carbide, chromiumoxide, ceriumoxide, α-iron oxide, corundum, artificial diamond, siliconnitride, silicon carbide, titanium carbide, titanium oxide, silicondioxide, and boron nitride. These may be used alone or in combination. Acomposite made up of two or more of these abrasive materials (e.g., oneobtained by surface-treating one abrasive material with another) mayalso be used. Although in some cases these abrasive materials containcompounds or elements other than the main component, the same effect isobtained with such abrasive materials as long as the content of the maincomponent is 90% or higher. These abrasive materials have a particlesize of preferably from 0.01 to 2 μm, more preferably from 0.05 to 1.0μm, most preferably from 0.05 to 0.5 μm. An abrasive material having anarrower particle size distribution is preferred especially forenhancing electromagnetic characteristics. For improving durability,abrasive materials having different particle sizes may be used incombination according to need. Alternatively, a single abrasive materialhaving a widened particle diameter distribution may be used so as toproduce the same effect. The abrasive material to be used preferably hasa tap density of from 0.3 to 2 g/ml, a water content of from 0.1 to 5%,a pH of from 2 to 11, and a specific surface area of from 1 to 30 m²/g.Although abrasive materials that can be used in the present inventionmay have any particle shape selected from the acicular, spherical, anddice forms, a particle shape having a sharp corner as part of thecontour is preferred because abrasive materials of this shape have highabrasive properties. Specific examples of abrasive materials usable inthe present invention include AKP-12, AKP-15, AKP-20, AKP-30, AKP-50,HIT 20, HIT-30, HIT-55, HIT 60, HIT 70, HIT 80, and HIT 100,manufactured by Sumitomo Chemical Co., Ltd.; ERC-DBM, HP-DBM,andHPS-DBMmanufacturedbyReynolds Co.; WA 10000 manufactured by FujimiKenmazai Kogyo K.K.; UB 20 manufactured by C. Uyemura & Co., Ltd.; G-5,Kromex U2, and Chromex U1 manufactured by Nippon Chemical IndustrialCo., Ltd.; TF 100 and TF 140 manufactured by Toda Kogyo Corp.; β-RandomUltrafine manufactured by Ibiden Co., Ltd.; and β-3manufactured by ShowaIndustry Co., Ltd. These abrasive materials may be added also to thenonmagnetic layer according to need. The incorporation of an abrasivematerial in the nonmagnetic layer has the effect of regulating thesurface shape or regulating the projecting state of an abrasivematerial. The particle diameter and amount of the abrasive material tobe added to each of the magnetic layer and the nonmagnetic layer should,of course, be optimized.

[0064] [Additives]

[0065] Additives having a lubricating, antistatic, dispersing,plasticizing, or another effect may be used in the magnetic layer andnonmagnetic layer in the present invention. Examples of such additivesinclude molybdenum disulfide, tungsten disulfide, graphite, boronnitride, fluorinated graphite, silicone oils, silicones having a polargroup, fatty acid-modified silicones, fluorine-containing silicones,fluorine-containing alcohols, fluorine-containing esters, polyolefins,polyglycols, esters of alkylphosphoric acids and alkali metal salts ofthe esters, esters of alkylsulfuric acids and alkali metal salts of theesters, poly(phenyl ether)s, phenylphosphonic acid, α-naphthylphosphoricacid, phenylphosphoric acid, diphenylphosphoric acid,p-ethylbenzenephosphonic acid, phenylphosphinic acid, aminoquinones,various silane coupling agents, titanium coupling agents,fluorine-containing alkylsulfuric acid esters and alkali metal salts ofthe esters, monobasic fatty acids having 10 to 24 carbon atoms (whichmay have an unsaturated bond or be branched) and metal salts (e.g.,lithium, sodium, potassium, or copper salts) of these acids, mono-, di-,tri-, tetra-, penta-, and hexahydric alcohols having 12 to 22 carbonatoms (which may have an unsaturated bond or be branched),alkoxyalcohols having 12 to 22 carbon atoms, mono-, di-, or triesters ofa monobasic fatty acid having 10 to 24 carbon atoms (which may have anunsaturated bond or be branched) with any one of mono-, di-, tri-,tetra-, penta-, and hexahydric alcohol shaving2to 12carbonatoms (whichmay have an unsaturated bond or be branched), fatty acid esters ofmonoalkyl ethers of alkylene oxide polymers, fatty acid amides having 8to 22 carbon atoms, and aliphatic amines having 8 to 22 carbon atoms.

[0066] Examples of the fatty acids include capric acid, caprylic acid,lauric acid, myristic acid, palmitic acid, stearic acid, behenicacid,oleicacid, elaidicacid, linoleicacid, linolenic acid, and isostearicacid. Examples of the esters include butyl stearate, octyl stearate,amyl stearate, isooctyl stearate, butyl myristate, octyl myristate,butoxyethyl stearate, butoxydiethyl stearate, 2-ethylhexyl stearate,2-octyldodecyl palpitate, 2-hexyldodecyl palpitate, isohexadecylstearate, oleyl oleate, dodecyl stearate, tridecyl stearate, erucic acidoleyl, neopentyl glycol didecanoate, and ethylene glycol dioleate.Examples of the alcohols include oleyl alcohol, stearyl alcohol, andlauryl alcohol. Surfactants are also usable. Examples thereof includenonionic surfactants such as those of the alkylene oxide type, glyceroltype, glycidol type, and alkylphenol ethylene oxide adduct type;cationic surfactants such as cyclic amines, ester amides, quaternaryammonium salts, hydantoin derivatives, heterocyclic compounds, andphosphonium or sulfonium compounds; anionic surfactants containing anacid radical such as a carboxylic acid, sulfonic acid, phosphoric acid,sulfate, or phosphate radical; and amphoteric surfactants such as aminoacids, aminosulfonic acids, esters of sulfuric or phosphoric acid withamino-alcohols, and alkylbetaines. These surfactants are described indetail in Kaimen Kasseizai Binran (published by Sangyo Tosho K.K.).These additives including lubricants and antistatic agents need not be100% pure, and may contain impurities such as isomers, unreactedproducts, by-products, decomposition products, oxidation products, etc.,besides the main components. The content of these impurities isdesirably 30% or lower, preferably 10% or lower.

[0067] Those lubricants and surfactants usable in the present inventionhave different physical functions, and the kinds and amounts of suchingredients and a lubricant proportion for producing a synergisticeffect should be optimized according to purposes. Lubricants orsurfactants may be used, for example, in the following manners: fattyacids having different melting points are used for the nonmagnetic layerand the magnetic layer, respectively, to control the bleeding to thesurface; esters differing in boiling point, melting point, or polarityare used to control the bleeding to the surface; surfactant amounts areregulated to improve the stability of coating; and a larger lubricantamount is used for an interlayer to obtain an improved lubricatingeffect. It is a matter of course that manners of using lubricants orsurfactants are not limited to these examples. In general, the totalamount of the lubricants to be used is selected from the range of from0.1 to 50%, preferably from 2 to 25%, based on the magnetic material orthe nonmagnetic particles.

[0068] Part or all of the additives to be used in the present inventionmaybe added at any step in the course of the production of a magneticcoating solution or nonmagnetic coating solution. Examples of additiontechniques include: to mix the additives with a magnetic material priorto a kneading step; to add the additives during the step of kneading amagnetic material together with a binder and a solvent; to add theadditives at a dispersing step; to add the additives after dispersion;and to add the additives immediately before coating. Some purposes maybe accomplished by forming a magnetic layer by coating and then applyingpart or all of one or more additives simultaneously or successivelyaccording to the purposes. It is also possible to apply a lubricant tothe surface of the magnetic layer after calendering or after completionof slitting, according to purposes. Well-known organic solvents can beused in the present invention. For example, the solvents shown inJapanese Patent Application (Laid-Open) No. 68453/1994 can be used.

[0069] [Layer Constitution]

[0070] In the layer constitution of the magnetic tape of the presentinvention, the support has a thickness of generally from 2 to 100 μm,preferably from 2 to 80 μm. For computer tapes, a support having athickness in the range of generally from 3.0to 6.5 μm (preferablyfrom3.0to 6.0 μm, morepreferably from 4.0 to 5.5 μm) is used.

[0071] An undercoat layer may be formed between the support and thenonmagnetic layer or magnetic layer for the purpose of improvingadhesion. This undercoat layer has a thickness of generally from 0.01 to0.5 μm, preferably from 0.02 to 0.5 μm. The recording medium of thepresent invention may be a tape-form medium produced by forming anonmagnetic layer and a magnetic layer on each side of a support, or maybe a tape-form medium produced by forming these layers on only one sideof a support. In this case, a back coat layer may be formed on the sideopposite to the nonmagnetic layer and magnetic layer for the purpose ofproducing an antistatic effect, anticurling effect, etc. This back coatlayer has a thickness of generally from 0.1 to 4 μm, preferably from 0.3to 2.0 μm. The undercoat layer and back coat layer each may be awell-known layer.

[0072] The nonmagnetic layer has a thickness of generally from 0.2to 5.0μm, preferably from 0.3 to 3.0 μm, more preferably from 1.0 to 2.5 μm.The nonmagnetic layer produces its effects as long as it issubstantially nonmagnetic. For example, even when the nonmagnetic layercontains a slight amount of a magnetic material either as an impurity oras an ingredient purposely incorporated, the effects of the presentinvention are brought about. It is a matter of course that suchconstitution can be regarded as substantially the same as that of thepresent invention. The term “substantially nonmagnetic” as used hereingenerally means that the residual magnetic flux density of thenonmagnetic layer is 0.01 T or lower or the coercive force thereof is7.96 kA/m (100 Oe) or lower, and preferably means that the nonmagneticlayer has no residual magnetic flux density and no coercive force.

[0073] [Back Coat Layer]

[0074] Compared to video tapes and audio tapes, the magnetic tapes forcomputer data recording generally are strongly required to have a highdegree of suitability for repetitions of running. For maintaining thehigh degree of running durability required, the back coat layerpreferably contains a carbon black and inorganic particles.

[0075] As the carbon black is preferably used a combination of two kindsof carbon blacks differing in average particle diameter. In this case, apreferred combination comprises a finely particular carbon black havingan average particle diameter of from 10 to 20 nm and a coarse carbonblack having an average particle diameter of from 230 to 300 nm.Addition of such a finely particulate carbon black is generallyeffective in regulating the back coat layer so as to have a low surfaceelectrical resistance and a low light transmittance. Since there aremany magnetic recording apparatus in which the light transmittance of atape is utilized as a signal for operation, the addition of a finelyparticulate carbon black is especially effective in such a case.Furthermore, finely particulate carbon blacks generally have the highability to hold a liquid lubricant and hence contribute to a reductionin coefficient of friction when a lubricant is used therewith incombination.

[0076] On the other hand, the coarse carbon black having an averageparticle diameter of from 230 to 300 nm not only functions as a solidlubricant but also forms minute projections on the surface of the backcoat layer to reduce the contact area and thereby contribute to areduction in coefficient of friction. However, use of the coarse carbonblack alone has a drawback that when the tape is used under severerunning conditions, carbon black particles are apt to fall down from theback coat layer due to tape sliding, leading to an increased errorfrequency.

[0077] Specific examples of commercial products of the finelyparticulate carbon black include the following. The average particlediameters thereof are shown in parentheses. RAVEN 2000B (18 nm) andRAVEN 1500B (17 nm) (manufactured by Columbian Carbon Co.); BP 800 (17nm) (manufactured by Cabot Corp.); PRINTEX 90 (14 nm), PRINTEX 95 (15nm), PRINTEX 85 (16 nm), and PRINTEX 75 (17 nm) (manufactured by DegussaAG); and #3950 (16 nm) (manufactured by Mitsubishi Chemical IndustriesLtd.).

[0078] Specific examples of commercial products of the coarse carbonblack include Thermal Black (270 nm) (manufactured by Cancarb Ltd.) andRAVEN MTP (275 nm) (manufactured by Columbian Carbon Co.).

[0079] When a finely particulate carbon black and coarse carbon blackwhich have average particle diameters of from 10 to 20 nm and from 230to 300 nm, respectively, are used in the back coat layer as two kinds ofcarbon blacks differing in average particle diameter, then theproportion (by weight) of the former to the latter carbon black ispreferably in the range of from 98:2 to 75:25, more preferably from 95:5to 85:15.

[0080] The amount of the carbon black (total carbon black amount whentwo or more carbon blacks are used) in the back coat layer is generallyfrom 30 to 80 parts by weight, preferably from 45 to 65 parts by weight,per 100 parts by weight of the binder.

[0081] The inorganic particles to be used preferably comprise acombination of two particulate materials differing in hardness. Forexample, it is preferred to use soft inorganic particles having a Mohs'hardness of from 3 to 4.5 and hard inorganic particles having a Mohs'hardness of from 5 to 9. Addition of the soft inorganic particles havinga Mohs' hardness of from 3 to 4.5 is effective in stabilizing thecoefficient of friction during repetitions of running. In addition, suchsoft inorganic particles, which have a hardness within that range, donot abrade sliding guide poles. These inorganic particles preferablyhave an average particle diameter in the range of from 30 to 50 nm.

[0082] Examples of the soft inorganic particles having a Mohs' hardnessof from 3 to 4.5 include calcium sulfate, calcium carbonate, calciumsilicate, barium sulfate, magnesium carbonate, zinc carbonate, and zincoxide. These may be used alone or in combination of two or more thereof.

[0083] The amount of the soft inorganic particles contained in the backcoat layer is in the range of preferably from 10 to 140 parts by weight,more preferably from 35 to 100 parts by weight, per 100 parts by weightof the carbon black.

[0084] Addition of the hard inorganic particles having a Mohs' hardnessof from 5 to 9 enhances the strength of the back coat layer to improverunning durability. When these inorganic particles are used incombination with a carbon black and the soft inorganic particlesdescribed above, the back coat layer has improved durability withreduced deterioration in repetitions of sliding. Furthermore, theaddition of the hard inorganic particles imparts a moderate abradingability to the layer to thereby diminish the adhesion of scratchedpowders to tape guide poles, etc. Especially when the hard inorganicparticles are used in combination with the soft inorganic particles, theback coat layer has improved sliding properties on guide poles having arough surface, whereby the coefficient of friction of the back coatlayer can also be stabilized.

[0085] The hard inorganic particles preferably have an average particlesize in the range of from 80 to 250 nm (more preferably from 100 to 210nm).

[0086] Examples of the hard inorganic particles having a Mohs' hardnessof from 5 to 9 include α-iron oxide, α-alumina, and chromium oxide(Cr₂O₃). These particulate materials may be used alone or in combinationof two or more thereof. Preferred of these are α-iron oxide andα-alumina. The amount of the hard inorganic particles contained in theback coat layer is generally from 3 to 30 parts by weight, preferablyfrom 3 to 20 parts by weight, per 100 parts by weight of the carbonblack.

[0087] When soft inorganic particles and hard inorganic particles are tobe used in combination in the back coat layer, these two kinds ofinorganic particles are preferably ones selected so that the differencein hardness between the soft inorganic particles and the hard inorganicparticles is 2 or more (more preferably 2.5 or more, most preferably 3or more) The back coat layer preferably contains the two particulateinorganic materials differing in Mohs' hardness and each having aspecific average particle size and further contains the two carbonblacks differing in average particle size.

[0088] A lubricant can be incorporated into the back coat layer. One ormore lubricants suitably selected from the lubricants enumerated aboveas lubricants usable in the nonmagnetic layer or magnetic layerdescribed above may be used. The amount of the lubricant to be added tothe back coat layer is generally from 1 to 5 parts by weight per 100parts by weight of the binder.

[0089] [Support]

[0090] The support used in the present invention is not particularlylimited. However, the support is preferably a substantially nonmagneticflexible support.

[0091] As the flexible support for use in the present invention can beemployed a well-known film. Examples thereof include films of polyesterssuch as poly(ethylene terephthalate) and poly (ethylene naphthalate)polyolefins, cellulose triacetate, polycarbonates, aromatic polyamides,aliphatic polyamides, polyimides, poly(amide-imide)s, polysulfones, andpolybenzoxazole. Preferredof these arehigh-strength supports made ofpoly(ethylene naphthalate), a polyamide, or the like. A laminate supportsuch as that described in Japanese Patent Application (Laid-Open) No.224127/1991 may be used according to need so that the magnetic layersurface and the base surface differ in surface roughness. These supportsmay be subjected beforehand to corona discharge treatment, plasmatreatment, easy adhesion treatment, heat treatment, dust-removingtreatment, etc. An aluminum or glass base may be also used as thesupport in the present invention.

[0092] Supports advantageously usable for accomplishing the aims of thepresent invention are ones which have a central-plane average surfaceroughness Ra as measured with TOPO-3D, manufactured by WYKO Corp., ofpreferably 8.0 nm or lower, more preferably 4.0 nm or lower, mostpreferably 2.0 nm or lower. Besides having such a low central planeaverage surface roughness, these supports are preferably free fromprojections as large as 0.5 μm or more. The state of the surfaceroughness can be freely controlled by regulating the size and amount ofa filler which is incorporated into the support according to need.Examples of the filler include oxides or carbonates of calcium, silicon,and titanium and fine organic powders such as acrylic powders. Thesupport preferably has a maximum height R_(max) of 1 μm or smaller, aten-point average roughness R_(a) of 0.5 μm or lower, a central planepeak height Rp of 0.5 R_(z) or smaller, a central plane valley depthR_(y) of 0.5 μm or smaller, a central plane areal ratio Sr of from 10 to90%, and an average wavelength % a of from 5 to 300 μm. The surfaceprojections on the support can be controlled with a filler so as to haveany desired distribution, for the purpose of obtaining the desiredelectromagnetic characteristics and durability. The number of surfaceprojections respectively having sizes of various ranges of from 0.01 pmto 1 μm can be regulated so as to be from 0 to 2,000 per 0.1 mm². Thesupport to be used in the present invention has an F-5 value ofpreferably from 5 to 50 kg/mm² (49 to 490 MPa). The degree of thermalshrinkage of the support as measured under the conditions of 100° C. and30 minutes is preferably 3% or lower, more preferably 1.5% or lower, andthe degree of thermal shrinkage thereof as measured under the conditionsof 80° C. and 30 minutes is preferably 1% or lower, more preferably 0.5%or lower. The strength at break of the support is preferably from 5 to100 kg/mm² (≈49 to 980 MPa), and the modulus of elasticity thereof ispreferably from 100 to 2,000 kg/mm² ({square root}0.98 to 19.6 GPa). Thethermal expansivity of the support is generally from 10⁻⁴ to 10⁻⁸/° C.,preferably from 10⁻⁵ to 10⁻⁶/° C., and the hygroexpansivity thereof isgenerally 10⁻⁴/H % or lower, preferably 10⁻ ⁵/RH % or lower. It ispreferred that the support be almost homogeneous in each of thesethermal, dimensional, and mechanical properties in such a degree thatthe difference in each property between any in-plane directions in thesupport is within 10%.

[0093] [Process for Production]

[0094] A process for producing a magnetic coating solution ornonmagnetic coating solution to be used for producing the magnetic tapeof the present invention comprises at least a kneading step and adispersing step, and may further comprise a mixing step which mayoptionally be conducted before and after the two steps. Each step mayinclude two or more stages. Each of the materials for use in the presentinvention, including a magnetic material, nonmagnetic particles, binder,carbon black, abrasive material, antistatic agent, lubricant, andsolvent, may be added in any step either at the beginning of or duringthe step. Furthermore, each raw material may be added portion-wise intwo or more steps. For example, a polyurethane may be added portion-wiseso that it is added in each of the kneading step, dispersing step, andmixing step for viscosity adjustment after the dispersion. Well-knownmanufacturing techniques can be used as part of the process in order toaccomplish the aims of the present invention. In the kneading step ispreferably used a kneading machine having a high kneading power, such asan open kneader, continuous kneader, pressure kneader, or extruder. Inthe case of using a kneader, the magnetic material or nonmagneticparticles are kneaded together with all or part (preferably at least30%) of the binder, the binder amount being in the range of from 15 to500 parts per 100 parts of the magnetic material. Details of thiskneading treatment are given in Japanese Patent Application (Laid-Open)Nos. 106338/1989 and 79274/1989. Although glass beads can be used forparticle dispersion in preparing coating solutions respectively forforming a magnetic layer and a nonmagnetic layer, it is preferred to usezirconia beads, titania beads, or steel beads, which are dispersingmedia having a high specific gravity. Such a dispersing medium optimizedin particle diameter is used in an optimal loading (i.e., a packingratio). Well-known dispersing machines can be used.

[0095] In the case where a magnetic tape having a multilayerconstitution according to the present invention is produced throughcoating, it is preferred to use any of the following methods.

[0096] The first method is to firstly form a lower layer through coatingwith a coating apparatus generally used for applying magnetic coatingsolutions, e.g., agravure coating, roll coating, blade coating, orextrusion coating apparatus, and then form an upper layer, while thelower layer is still in a wet state, through coating with thesupport-pressing extrusion coater disclosed in Japanese PatentPublication No. 46186/1989 or Japanese Patent Application (Laid-Open)No. 238179/1985 or 265672/1990.

[0097] The second method is to almost simultaneously form an upper layerand a lower layer through coating with a single coating head havingtherein two slits for passing coating solutions, such as those disclosedin Japanese PatentApplication (Laid-Open) Nos. 88080/1988, 17971/1990,and 265672/1990.

[0098] The third method is to almost simultaneously form an upper layerand a lower layer through coating with the extrusion coater equippedwith a back-up roll as disclosed in Japanese Patent Application(Laid-Open) No. 174965/1990.

[0099] For preventing the electromagnetic characteristics and otherproperties of the magnetic tape from being impaired by cohesion ofmagnetic particles, it is desirable to apply a shearing force to thecoating solution in the coating head by amethod such as those disclosedin Japanese PatentApplication (Laid-Open) Nos. 95174/1987 and236968/1989. The viscosity of each coating solution should be in therange specified in Japanese Patent Application (Laid-Open) No.8471/1991.

[0100] For realizing a multilayer constitution, successive multiplecoating may, of course, be used in which a coating solution is appliedand dried to form a lower layer and a magnetic layer is then formedthereon. Even with this coating technique, the intact effects of thepresent invention are obtained. However, from the standpoint ofdiminishing coating defects to improve quality such as freedom fromdropouts, it is preferred to use any of the techniques for simultaneousmultiple coating described above.

[0101] In an orientation treatment of the magnetic layer, a cobaltmagnet or solenoid is used to orient the magnetic particles in themachine direction. It is preferred that the place in which the coatingfilm is dried be made controllable by regulating the temperature andamount of the air fed for drying and the rate of coating. The rate ofcoating is preferably from 20 to 1,000 m/min and the temperature of thedrying air is preferably 60° C. or higher. Predrying may be performed toan appropriate degree before the coated support enters the magnet zone.

[0102] After the coating and drying, the magnetic tape is usuallysubjected to a calendering treatment. In the calendering treatment, thecoated support is treated with plastic calendering rolls made of aheat-resistant plastic, e.g., an epoxy, polyimide, polyamide, orpoly(imide-amide), or with metallic calendering rolls. Especially whenthe support has a magnetic layer on each side, the coated support ispreferably calendered between metal rolls. The calendering temperatureis preferably 50° C. or higher, more preferably 100° C. or higher. Thelinear pressure is preferably 200 kg/cm (196 kN/m) or higher, morepreferably 300 kg/cm (294 kN/m) or higher.

[0103] [Physical Properties]

[0104] The saturation flux density of the magnetic layer in the presentinvention is preferably from 0.1 to 0.3 T. The machine directioncoercive force H_(c) of the magnetic layer is as stated hereinabove;narrower coercive force distributions are preferred, with the SFD beingpreferably 0.6 or lower.

[0105] The coefficient of friction of the magnetic tape of the presentinvention with heads is generally 0.5 or lower, preferably 0.3 or lower,in the temperature range of from −10° C. to 40° C. and the humidityrange of from 0 to 95%. The intrinsic surface resistivity of themagnetic tape on the magnetic layer side is preferably from 10⁴ to 10₁₂Ωg/sq, and the electrification potential of the tape is preferably from−500 to +500 V.

[0106] The modulus of elasticity at 0.5% elongation of the magneticlayer is preferably from 100 to 2,000 kg/mm² (0.98 to 19.6 GPa) in anyin-plane direction, and the strength at break thereof is preferably from10 to 70 kg/mm² (98 to 686 MPa). The modulus of elasticity of themagnetic tape is preferably from 100 to 1,500 kg/mm² (0.98 to 14.7 GPa)in any in-plane direction, the residual elongation thereof is preferably0.5% or lower, and the degree of thermal shrinkage thereof as measuredat any temperature not higher than 100° C. is preferably 1% or lower,more preferably 0.5% or lower, most preferably 0.1% or lower. The glasstransition temperature (the temperature corresponding to a maximum ofloss modulus in a dynamic viscoelasticity measurement made at 110 Hz) ofthe magnetic layer is preferably from 50 to 120° C., while that of thelower nonmagnetic layer is preferably from 0 to 100° C. The loss modulusis preferably in the range of from 1×10⁹ to 8×10¹⁰ μN/cm² and the losstangent is preferably 0.2 or smaller. Too large loss tangents tend toresult in sticking troubles. It is preferred that the medium be almosthomogeneous in each of these thermal and mechanical properties in such adegree that the difference in each property between any in-planedirections in the medium is within 10%. The residual solvent content inthe magnetic layer is preferably 100 mg/m or lower, more preferably 10mg/m² or lower. The void content in each of the coating layers, i.e.,the nonmagnetic layer and magnetic layer, is preferably 30% by volume orlower, more preferably 20% by volume or lower. Although a lower voidcontent is desirable for attaining higher output, there are cases wherea certain degree of void content is advantageous for some purposes. Forexample, in the case of disk media for which suitability for repetitionsof running is important, higher void contents frequently bring aboutbetter running durability.

[0107] The central plane average surface roughness R_(a) of the magneticlayer as measured with TOPO-3D, manufactured by WYKO Corp., over an areaof about 250 μm×250 μm is generally 4.0 nm or lower, preferably 3.8 nmor lower, more preferably 3.5 nm or lower. The magnetic layer preferablyfurther has a maximum height R_(max) of 0.5 μm or smaller, a ten-pointaverage roughness R_(z) of 0.3 μm or lower, a central plane peak heightR_(p) of 0.3 μm or smaller, a central plane valley depth R_(v) of 0.3 μmor smaller, a central plane areal ratio S_(r) of from 20 to 80%, and anaverage wavelength λ_(a) of from 5 to 300 μm. The surface projectionspresent on the magnetic layer are preferably regulated so as to meetthose surface properties to thereby optimize the electromagneticcharacteristics and coefficient of friction of the layer. The state ofthese surface projections can be easily controlled by controlling thesurface state of the support with a filler and by regulating theparticle diameter and amount of the filler to be added to the magneticlayer as stated above or regulating the surface shape of the rolls to beused for calendering, etc. The curling of the magnetic tape ispreferably regulated so as to be within ±3 mm.

[0108] In the case where the magnetic tape of the present invention hasa nonmagnetic layer and a magnetic layer, it can be made to have adifference in any of those physical properties between the nonmagneticlayer and the magnetic layer according to purposes, a scan be easilypresumed. For example, the magnetic layer is made to have a heightenedmodulus of elasticity to improve running durability and, at the sametime, the nonmagnetic layer is made to have a lower modulus ofelasticity than the magnetic layer to improve the head touching of themagnetic tape.

EXAMPLES

[0109] The present invention will be explained below in more detail byreference to Examples, but the present invention should not be construedas being limited thereto. In the following Examples and ComparativeExamples, all “parts” are by weight.

Example 1

[0110] <Preparation of Coating Solutions> (Magnetic Coating Solution)Ferromagnetic particles: magnetic barium ferrite 100 parts particles(BaFe) Average tabular diameter, 30 nm; average tabular thickness, 10nm; average particle volume, 5,800 nm³; H_(c), 183 kA/m; σ_(s), 50 A ·m²/kg; S_(BET), 65 m²/g Vinyl chloride copolymer  10 parts MR 110(manufactured by Nippon Zeon Co., Ltd.) Polyurethane resin  5 partsT_(g) = 82° C. α-Alumina  5 parts HIT 55 (manufactured by SumitomoChemical Co., Ltd.) Particle size, 0.2 μm Carbon black  1 part #55(manufactured by Asahi Carbon Co., Ltd.) Average primary particlediameter: 0.075 μm Specific surface area: 35 m²/g DBP absorption amount:81 mL/100 g pH: 7.7 Volatile content: 1.0% Butyl stearate  10 partsButoxyethyl stearate  5 parts Isohexadecyl stearate  3 parts Stearicacid  2 parts Methyl ethyl ketone 125 parts Cyclohexanone 125 parts(Nonmagnetic Coating Solution) Nonmagnetic particles; acicular hematite 80 parts Average long-axis length: 0.15 μm Acicular ratio: 7 BETspecific surface area: 50 m²/g pH: 8.5 Surface layer treated: Al₂O₃Carbon black  20 parts Average particle diameter: 20 nm Vinyl chloridecopolymer  7 parts MR 110 (manufactured by Nippon Zeon Co., Ltd.)Polyurethane resin  10 parts T_(g): 55° C. Butyl stearate  1 partStearic acid  3 parts Methyl ethyl ketone/cyclohexanone 250 parts (8/2mixed solvent)

[0111] <Production of Computer Tape>

[0112] With respect to each of the coating solutions shown above, theingredients were kneaded with a kneader and the resultant mixture wastreated with a sand mill for 4 hours to disperse the particulateingredients. To the resultant dispersions was added a polyisocyanate inan amount of 2.5 parts for the coating solution for nonmagnetic layerformation and in an amount of 3 parts for the coating solution formagnetic layer formation. Forty parts of cyclohexanone was further addedto each dispersion. The resultant mixtures were filtered through afilter having an average pore diameter of 1 μm. Thus, coating solutionsfor nonmagnetic layer formation and magnetic layer formation,respectively, were prepared.

[0113] An aramid support having a thickness of 4.4 μm and a centralplane average surface roughness of 2 nm was coated by simultaneousdouble coating in the following manner. The coating solution fornonmagnetic layer formation was first applied in such an amount as togive a lower layer having a thickness of 1.7 μm on a dry basis, and thecoating solution for magnetic layer formation was simultaneously appliedthereon in such an amount as to give a magnetic layer having a thicknessof 100 nm on a dry basis. While the two layers were still wet, themagnetic particles were oriented with a cobalt magnet having a magneticforce of 0.6 T and a solenoid having a magnetic force of 0.6 T. Afterbeing dried, the web was calendered with a 7-roll calender in which allthe rolls were metal rolls. The calendering was conducted at atemperature of 85° C. and a calendering speed of 200 m/min. Thereafter,a back layer having a thickness of 0.5 μm was formed by applying acoating solution (prepared by dispersing 100 parts of carbon blackhaving an average particle size of 17 nm, 80 parts of calcium carbonatehaving an average particle size of 40 nm, and 5 parts of α-aluminahaving an average particle size of 200 nm in a nitrocelluloseresin/polyurethane resin/polyisocyanate mixture).

[0114] The web was then slit into a ½ inch width, and the surface of themagnetic layer thereof was cleaned with a tape-cleaning apparatusobtained by modifying a slit-film unwinding/winding apparatus byattaching a nonwoven fabric and a razor so as to be pressed against themagnetic layer surface. Thus, a computer tape sample was obtained.

[0115] <Evaluation Methods>

[0116] The computer tape was evaluated for the following performances bythe methods shown below. The results obtained are shown in Table 1.

[0117] (1) Magnetic Properties

[0118] H_(c), SQ⊥: Measurement was made with a vibrating samplemagnetometer (manufactured by Toei Kogyo) at an H_(m) of 10 kOe (≈796kA/m). H_(c) was measured in the tape machine direction, while SQ⊥ wasmeasured in the direction perpendicular to the magnetic plane.

[0119] (2) S/N Ratio

[0120] Measurement was made with an apparatus obtained by modifying tapedrive LTO Ultrium, manufactured by IBM. The recording current was set atthe optimal value for each tape. For determining S/N ratio, signalshaving a wavelength of 0.3 μm were recorded, and the signals reproducedwere analyzed for frequency with a spectrum analyzer manufactured byShibaSoku Co., Ltd. The ratio of the output of the carrier signals(wavelength, 0.3 μm) to the integrated noise for the whole spectralregion was taken as the S/N ratio.

Examples 2 to 5 and Comparative Examples 1 to 4

[0121] Computer tape samples were produced and evaluated in the samemanner as in Example 1, except that the magnetic materials shown inTable 1 were used in forming a magnetic layer. In Table 1, the Fe—Coindicates an Fe—Co alloy, and the average tabular diameter and averagetabular thickness mean average long-axis length and average short-axislength, respectively. The results obtained are shown in Table 2. Thesample of Comparative Example 1 was a tape employing the Fe—Co alloy,and was used as a reference. In Comparative Example 2, TiO₂ (averageparticle diameter, 35 nm) was used as the nonmagnetic particles for thenonmagnetic lower layer. In Comparative Example 3, no nonmagnetic lowerlayer was formed. TABLE 1 Kind of magnetic material BaFe1 BaFe2 BaFe3BaFe4 BaFe5 Fe—Co Average 30 15 38 30 45 100 tabular diameter (nm)Average 10 7 12 10 15 15 tabular thickness (nm) Average 5800 1100 120005800 19500 17700 particle volume (nm³) H_(c) [kA/m] 183 155 185 300 130146 σ_(s) (Am²/kg) 50 46 53 50 55 125 S_(BET) (m²/g) 65 87 52 65 43 56

[0122] TABLE 2 Example Comparative Example 1 2 3 4 5 1 2 3 4 Kind ofmagnetic BaFe BaFe BaFe BaFe BaFe Fe—Co BaFe BaFe BaFe material 1 2 3 14 1 1 5 Magnetic layer H_(c) [kA/m] 190 162 190 191 330 146 190 190 138SQ⊥ 0.3 0.41 0.18 0.46 0.3 0.2 0.53 0.6 0.32 Thickness 100 100 100 28040 100 100 340 100 [nm] Nonmagnetic layer pre- pre- pre- pre- pre- Pre-pre- ab- pre- sent sent sent sent sent sent sent sent sent S/N (forward)[dB] 26 27 24.5 23 24.9 17.2 23.6 20.5 17.6 S/N (reverse) [dB] 24.5 25.324.1 21.3 23.7 16.8 19.4 15.5 16.3 S/N difference 1.5 1.7 0.4 1.7 1.20.4 4.2 5.0 1.3

[0123] The results given in Table 2 show the following.

[0124] The tapes of Examples 1 to 5 according to the present inventioneach had satisfactory electromagnetic characteristics with an excellentS/N ratio in each of forward and reverse operations.

[0125] On the other hand, the tape of Comparative Example 2 had anincreased value of SQ⊥ and a reduced amount of magnetizable componentsin the machine direction because of the use of TiO₂ in the lower layer,resulting in a reduced S/N ratio (reverse) and an increased S/Ndifference. The tape of Comparative Example 3, which had no lower layer,had a further increased value of SQ⊥ and had no cushioning effect.Consequently, the tape of Comparative Example 3 had an even lower S/Nratio and a larger S/N difference. The tape of Comparative Example 4 hada reduced S/N ratio because the magnetic layer had too low a value ofH_(c) due to the large magnetic-layer thickness and the large particlesize of the BaFe.

[0126] The magnetic tape of the present invention, when used in arecording/reproducing system employing an MR head, is reduced in noiseand has a reduced difference in noise level betweenrecording/reproducing directions. Consequently, the magnetic tape hasexcellent suitability for high-density recording in the serpentine mode.Moreover, since the magnetic tape of the present invention is of thecoated type, it has excellent productivity.

[0127] The entitle disclosure of each and every foreign patentapplication from which the benefit of foreign priority has been claimedin the present application is incorporated herein by reference, as iffully set forth herein.

[0128] While the invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

What is claimed is:
 1. A magnetic tape comprising a support havingthereon a magnetic layer comprising ferromagnetic particles and a binderas the main components, (i) the magnetic tape being for use in amagnetic recording/reproducing system in which signals recorded arereproduced with a magneto resistive head, (ii) the ferromagneticparticles contained in the magnetic layer being hexagonal-ferritemagnetic particles having an average tabular diameter of from 10 to 40nm, and the magnetic layer having a machine direction coercive forceH_(c) of from 158 to 350 kA/m, a squareness ratio SQ⊥ as measured in thedirection perpendicular to the magnetic plane of 0.5 or lower, and athickness of from 30 to 300 nm.
 2. The magnetic tape as in claim 1,which is for use in recording/producing in the linear serpentine mode.3. The magnetic tape as in claim 1, wherein the magnetic layer is formedon the support either directly or through a nonmagnetic lower layer. 4.The magnetic tape as in claim 1, which has a multilayer constitutionincluding a nonmagnetic lower layer.
 5. The magnetic tape as in claim 1,wherein the machine direction coercive force H_(c) of the magnetic layeris from 170 to 280 kA/m.
 6. The magnetic tape as in claim 1, wherein themagnetic layer has a magnetization distribution that the amount ofcomponents which undergo a (magnetic) flux revolution upon applicationof a magnetic field of 80 kA/m or lower is smaller than 1%.
 7. Themagnetic tape as in claim 1, wherein the magnetic layer has a thicknessof from 50 to 250 nm
 8. The magnetic tape as in claim 1, wherein themagnetic layer has a squareness ratio SQ as measured in an in-planedirection is from 0.6 to 0.95
 9. The magnetic tape as in claim 1,wherein the magnetic layer has a squareness ratio SQ⊥ as measured in thedirection perpendicular to the magnetic plane is 0.4 or lower.